This invention relates to a ducted fan propulsion unit for aircraft and being of the kind wherein the fan is adapted to deliver air rearwards in respect of the aircraft to exert a forward thrust thereon or forwards in respect of the aircraft to exert a braking effect thereon.
British Pat. No. 1,544,826 discloses a ducted fan propulsion unit for aircraft comprising an annular main duct, a fan contained within the main duct and having blades the pitch of which is reversible, a first opening defined in the duct forward of the fan, an annular flow splitter situated rearward of the fan and dividing the main duct into radially outer and inner branch ducts, a passage extending radially through the splitter to connect the branch ducts, outer stator vanes provided in the outer branch duct rearward of said passage, inner stator vanes provided in the inner branch duct forward of said passage, a second opening defined in the outer branch duct rearward of the stator vanes therein, wherein the part of the inner branch duct lying forward of the passage has a cylindrical radially outer surface defined on the splitter and a radially inner surface whose diameter is progressively reducing in the rearward direction, and wherein the part of the inner duct rearward of said passage is defined by outer and inner surfaces both of progressively rearwardly reducing diameter and leading to a compressor of a gas turbine engine for driving the fan.
In one pitch setting of the fan blades, this being the forward thrust setting, air is sucked by the fan through the first opening and passes rearwards into the outer and inner branch ducts. In another pitch setting of the fan blades, this being the reverse thrust setting, the air is sucked by the fan through the second opening into the outer duct and passes from there in part forwards to the first opening and in part through said passage and then rearwards through the part of the inner duct rearward of the passage.
It has been found that in the forward thrust setting there occurs aerodynamic instability of the air passing through the forward part of the inner branch duct. In particular, the air tends to break away from said radially inner surface of said forward part. This is basically due to the fact that said forward part is divergent by reason of its radially outer surface being cylindrical while its inner surface is of rearwardly reducing diameter. Said instability is accentuated by the presence of said stator vanes in said forward part. The problem is particularly acute if one seeks to reduce the overall length of the powerplant because this has the effect, as far as said forward part is concerned, of increasing the rate at which the diameter of said radially inner surface decreases. It is an object of this invention to reduce or overcome that difficulty.